Peavey Delta Blues hum. please help!

ok, i have a problem. i just picked up a DB with the 15" speaker about a month ago. about two weeks ago, it started to have this nasty hum with the guitar volumes up. the amp sits quiet with the volume rolled off, but when i turn it up it hums at around 30% as loud as the actual guitar volume. (it hums on both clean and overdrive)

i've tried it with a friends gretsch country gentleman as well as my les paul and tele. its there with all of them on multiple cables, so i think that i can rule out it just being the guitar.

I replaced the stock tubes with JJs in hope that they would rectify the problem, but no luck. I'm figuring i could probably fix the whole thing with an electroharmonix Hum Debugger, but i was hoping for a cheaper fix. any ideas what the problem could be? help is appreciated. thanks

Is the reverb working ? Is the reverb turned up ? If so turn it off and try because the reverb is a big contributor to hu8m in this amp. Could be the reverb op-amp. If one tube has lost it's bias as in a bad or cold solder joint or any ground connection as Enzo pointed out could do it. Of course thinking the worst a power tube short could have taken something out causing a failure you may not detect without taking the boards out and further examining and testing.

what i mean with the quitar volume up is that it goes away when i roll the onboard volume off on the guitar, the hum leaves. bring the guitar volume up again and it returns. it happens on humbuckers and single coils. was happening before i switched tubes as well. (one of the reasons i swapped them out) hums with or without reverb engaged.

im going to call the shop tomorrow and see about getting it looked at. it has a two year warranty, so that should be helpful.

could it still be tubes after having switched them? i dont really want to have to take it in, but i feel im too inept to crack open the amp and tinker. (that and my warranty goes away if i choose that path)